Invasive infections due to Aspergillus fumigatus are increasing, and despite new therapies are associated with greater than 50% mortality. During the pathogenesis of invasive aspergillosis, the organism interacts with several types of host cells, including pulmonary epithelial cells and endothelial cells. After being inhaled, A. fumigatus conidia adhere to and are internalized by the pulmonary epithelial cells that line the alveoli. In susceptible hosts, these conidia then germinate to form hyphae, which exit the pulmonary epithelial cells and invade the deeper tissues, most notably pulmonary blood vessels. Within these blood vessels, hyphae interact with vascular endothelial cells in two unique ways. First, hyphae from an infectious focus invade the abluminal surface of the blood vessel and then penetrate endothelial cells to gain access to the blood vessel lumen. Next, hyphal fragments are borne by the bloodstream to distal sites where they adhere to and penetrate the luminal surface of the endothelial cells and then invade the deep organs. To date, little is known about the genetic control of A. fumigatus virulence traits that influence the interactions of the organism with these host cells. In other pathogenic fungi, the conserved signal transduction pathways that control fungal morphogenesis and growth are also key regulators of virulence traits. Data from our group and others suggest that this paradigm holds true for A fumigatus. Thus, we hypothesize that the interactions of A fumigatus with pulmonary epithelial cells and the vascular endothelium are both developmental^ regulated and critical to the pathogenesis of invasive aspergillosis. Towards this end, we have identified a number of A fumigatus transcription factors predicted to govern fungal development. Mutants that are deficient in these candidate genes have been constructed. We have also developed animal and in vitro model systems to investigate the interaction of these mutants with epithelial and endothelial cells in a physiologically relevant manner. We hypothesize that these mutants will have abnormal development, aberrant interactions with host cells, and attenuated virulence. We will test this hypothesis by 1) determining the role of candidate transcription factors in the development and morpho- genesis of A fumigatus;2) investigating the interactions of mutants deficient in candidate transcription factors with pulmonary epithelial cells and vascular endothelial cells in vitro;3) characterizing the ability of a subset of these candidate transcription factors to modulate virulence and host response using murine models of invasive aspergillosis;and 4) identifying the downstream genes whose expression is regulated by candidate transcription factors that we find govern host cell interactions and virulence. These investigations will establish the roles of previously unexplored A. fumigatus transcription factors in governing development, host cell interactions, and virulence. They will also identify new genes that specify clearly defined, biologically relevant A fumigatus virulence traits. Collectively, these results will form the foundation for the future development of new therapeutic strategies for invasive aspergillosis.